The commercial value of field corn as a source of food and other products owes in large part to the development of hybrid seed. Virtually all corn grown commercially in the United States is planted from hybrid seed. Hybrid corn is generally understood to be the first generation of a cross between inbred lines, which have been developed by controlled self-pollination continued for several generations.
Inbred corn lines have genetically pure or homozygous genotypes. They will generally have desirable agronomic characteristics and be relatively resistant to insect and disease attacks, but will usually display reduced vigor and yield. Provided that the inbred parent lines are not closely related, corn hybrids will generally exhibit heterosis or hybrid vigor, i.e., which includes, yield greater than expected from either parent inbred line.
Typically, a commercially valuable inbred corn line results from plants that have been self-pollinated and selected for type for many generations. The plants become homozygous at almost all gene loci. Meiosis yields gametes of uniform genotype, and they produce a uniform population of true breeding progeny. A cross between two homozygous, inbred parent lines, each line producing uniform gametes, produces a uniform population of hybrid plants that may be heterozygous for many gene loci.
The development of high yielding corn hybrids that are agronomically sound based on stable inbred lines involves three steps: (1) the selection of plants from various germplasm pools; (2) the selfing of the selected plants for several generations to produce a series of parent inbred lines, each, although distinct from the others, breeding true and highly uniform, ; and (3) crossing the selected parent inbred lines with unrelated parent inbred lines to produce the hybrid progeny (F.sub.1).
During the inbreeding process in corn, the vigor of the line decreases. Vigor is restored when two unrelated inbred lines are crossed to produce the hybrid progeny (F1). An important consequence of the homozygosity and homogeneity of the parent inbred lines is that the hybrid between any two parent inbred lines will always be substantially the same. Once the parent inbred lines that give a superior hybrid have been identified, the hybrid seed can be reproduced indefinitely as long as the homogeneity of the inbred parent lines is maintained.
The breeder develops superior, inbred parental lines for producing hybrids by identifying and selecting in the progeny of diverse parents rare individuals having a desired phenotype or combination of traits based on a unique genetic endowment, which the breeder attempts to preserve by controlled inbreeding.